CVE-2021-40895
📋 TL;DR
CVE-2021-40895 is a Regular Expression Denial of Service (ReDoS) vulnerability in todo-regex v0.1.1 that allows attackers to cause denial of service by providing specially crafted invalid TODO statements. This affects applications using the vulnerable todo-regex library to parse TODO comments. The vulnerability can cause excessive CPU consumption and application unresponsiveness.
💻 Affected Systems
- todo-regex
📦 What is this software?
Todo Regex by Todo Regex Project
⚠️ Risk & Real-World Impact
Worst Case
Complete application denial of service, causing system unavailability and potential cascading failures in dependent services.
Likely Case
Degraded application performance, increased response times, and temporary service disruption until the malicious input stops.
If Mitigated
Minimal impact with proper input validation, rate limiting, and monitoring in place to detect and block malicious patterns.
🎯 Exploit Status
Exploitation requires sending specially crafted TODO statements to applications using the vulnerable library. The exploit is simple to implement once the vulnerable pattern is understood.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: v0.1.2 or later
Vendor Advisory: https://github.com/yetingli/SaveResults/blob/main/js/todo-regex.js
Restart Required: Yes
Instructions:
1. Update todo-regex to version 0.1.2 or later using npm update todo-regex. 2. Restart any applications using the library. 3. Verify the update was successful by checking the package.json file.
🔧 Temporary Workarounds
Input Validation and Sanitization
allImplement input validation to reject malformed TODO statements before they reach the vulnerable regex parser.
// Example JavaScript input validation
function validateTodoInput(input) {
const maxLength = 1000;
const validPattern = /^TODO:\s.*$/;
return input.length <= maxLength && validPattern.test(input);
}
Rate Limiting
allImplement rate limiting on endpoints that process TODO statements to prevent automated exploitation attempts.
// Example using express-rate-limit
const rateLimit = require('express-rate-limit');
const limiter = rateLimit({
windowMs: 15 * 60 * 1000, // 15 minutes
max: 100 // limit each IP to 100 requests per windowMs
});
app.use('/api/todo', limiter);
🧯 If You Can't Patch
- Implement Web Application Firewall (WAF) rules to block requests containing suspicious TODO statement patterns
- Monitor CPU usage and implement alerting for abnormal spikes that could indicate exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check package.json or package-lock.json for todo-regex version 0.1.1. Run: npm list todo-regexCheck Version:
npm list todo-regexVerify Fix Applied:
Verify todo-regex version is 0.1.2 or later. Run: npm list todo-regex | grep todo-regex📡 Detection & Monitoring
Log Indicators:
- High CPU usage spikes
- Application timeouts when processing TODO-related requests
- Multiple failed parsing attempts with similar patterns
Network Indicators:
- Unusually large number of requests to TODO processing endpoints
- Requests containing malformed TODO statements with repeating patterns
SIEM Query:
source="application.logs" AND ("TODO:" AND "timeout" OR "high cpu") | stats count by src_ip